The present invention relates to an apparatus for detecting the level of a low mass number element material within an enclosed vessel using neutron scattering. In U.S. Pat. No. 3,716,711 there is disclosed an apparatus using the technique of neutron scattering to detect the liquid level within an enclosed vessel. While the apparatus disclosed shows a neutron source and a detector for the slow neutrons that are scattered by the hydrogen of the liquid, it is deficient in several respects. For example, the apparatus is relatively bulky due to its construction, and not readily handled by the operator. In addition, no means are provided for protecting the operating personnel from exposure to excessive radiation. This is particularly true when the instrument is transported, since the one side of the instrument is designed so that the fast neutrons from the source may pass through the instrument and the wall of the vessel that encloses the liquid. Obviously, if the source of fast neutrons is made small enough so that the danger of exposure to radiation is reduced to acceptable levels, the ability of the instrument to detect the liquid level in heavy walled vessels will also be reduced. The patent does not describe or illustrate any means for solving this problem.